1. Field of the Invention
The present invention relates to a zoom lens, and in particular, to a three-unit zoom lens which is suitable for a compact camera, small in size, low in cost, and wide in field angle.
2. Description of Related Art
Two-unit zoom lenses which are simple in structure have been often used as zoom lenses for compact cameras. However, in such a zoom lens for compact cameras, demands on a compact design and a high variable magnification ratio have recently increased. In particular, where the magnification of an optical system is changed, the demand on a wide field angle at the wide-angle position has increased. Thus, with the two-unit zoom lens which is simple in structure, there is a limit to the maintenance of a good balance of aberration, extending from the wide-angle position to the telephoto position. As such, a three-unit zoom lens has been chiefly used.
In compact cameras, telephoto-type lens systems in which the last lens unit has negative power are often employed. This is because such a lens system for compact cameras, in contrast with a lens system for single-lens reflex cameras, has the advantage that there is little need to increase the back focal distance or the overall length can be reduced. In particular, in the three-unit zoom lens, the arrangement of lenses with positive, positive, and negative powers is widely used.
Also, in addition to satisfying these demands, a further-cost reduction has been required.
Conventional three-unit zoom lenses are variously proposed by Japanese Patent Kokai Nos. Hei 2-51116, Hei 5-88085, and Hei 7-120677.
A three-unit zoom lens disclosed in Kokai No. Hei 2-51116 has the arrangement of a small number of lens elements. However, the variable magnification ratio fails to reach 2, which is unsatisfactory in view of the high variable magnification ratio. Furthermore, since a gradient index lens is used to correct aberration, fabrication is difficult and a cost reduction cannot be achieved.
A three-unit zoom lens disclosed in Kokai No. Hei 5-88085 uses a small number of lens elements to achieve a wide field angle. However, since the overall length is long and particularly the telephoto ratio at the telephoto position is as high as 1.2, it is difficult to achieve compactness.
A zoom lens disclosed in Kokai No. Hei 7-120677 offers a cost reduction with a small number of lens elements, but is unsatisfactory for the demand on the wide field angle. A zoom lens set forth in Embodiment 5 of this publication has the wide field angle, but shows a high telephoto ratio at the telephoto position, and thus it is unsatisfactory for achieving compactness.
It is, therefore, an object of the present invention to provide a three-unit zoom lens which is small in size, low in cost, and wide in field angle, but offers favorable performance with a small number of lens elements by properly arranging individual lens units.
In order to accomplish the above object, the three-unit zoom lens in a first aspect of the present invention includes, in order from the object side, a first lens unit with positive refracting power, a second lens unit with positive refracting power, and a third lens unit with negative refracting power. When the magnification of the zoom lens is changed, extending from the wide-angle position to the telephoto position, individual lens units of the three-unit zoom lens are moved toward the object side so that a space between the first lens unit and the second lens unit, after being increased as the lens units are moved from the wide-angle position toward the proximity of the middle position, is decreased as they are moved toward the telephoto position, and a space between the second lens unit and the third lens unit is also decreased accordingly. In this case, the three-unit zoom lens has a variable magnification ratio of 2 or higher and the first lens unit is constructed with a single lens element.
The three-unit zoom lens in a second aspect of the present invention includes, in order from the object side, a first lens unit with positive refracting power, a second lens unit with positive refracting power, and a third lens unit with negative refracting power. When the magnification of the zoom lens is changed, extending from the wide-angle position to the telephoto position, spaces between individual lens units are changed. In this case, the first lens unit is constructed with a single lens element of positive refracting power, having a concave surface directed toward the object side.
In the three-unit zoom lens in the first or second aspect of the present invention, the first lens unit has at least one aspherical surface and is constructed of plastic.